I use Mac MS Office all the time to create csv files to import into civi without issue.
Civi parses the file looking for the correct format. It doesn't care what the file name is. So for some reason the csv file isn't either saving correctly or has extra things in it. Try saving the file as a csv. Then open it with TextEdit and look at the file. It should be something like this
ID,First Name,Last Name,Street Address
1,John,Smith,123 Easy Street
2,Jane,Jones,1 Main Street
You may or may not have the first row with the column names and the data between the commas may be inclosed in quotes.
If you file looks ok then you might need to look for hidden charaters.
You could also try the other csv formats. I see a couple of MS style cvs formats further down the list in the save as file type.
Other possibilities are the character set. The code has UTF8 all through it but I think other work.